1. Technical Field
The present invention relates to a headlight optical axis adjustment device also referred to as “auto levelizer” used in vehicles such as an automobile.
2. Related Art
According to provision of safety standard (Department of Transportation Law No. 67, Jul. 28, 1951) of Road Trucking Vehicle Law, vehicles such as automobiles (hereinafter referred to as a vehicle) must be provided with a “running headlight” (so-called high beam) and a “passing headlight” (so-called low beam), or a “light distribution variable headlight” which is a combination of the above (combination of high beam and low beam).
The “running headlight” is provided to check obstructions in terms of traffic at a front side of the vehicle at night, and the “passing headlight” is provided, in addition to such requirement, such that an irradiating light beam does not inhibit other traffics (i.e., not to dazzle the drivers of oncoming vehicles and pedestrians).
Although such “running headlight” and “passing headlight” are respectively independent lighting tools, most of the recent vehicles include the “light distribution variable headlight”, that is, a headlight for automatically adjusting luminosity of the irradiating light beam and spatial distribution in a relevant direction according to a traveling state at night, and thus the description will be made herein using the “light distribution variable headlight” by way of example. It should be noted that such lighting tools are not distinguished in the concept of the invention.
A driver switches the optical axis of the headlight to either “high beam” or “low beam” depending on the situation when traveling the vehicle at night. Specifically, the driver switches to “high beam” to have the optical axis of the headlight to an upper side (more precisely, horizontal) so as to be able to see sufficiently well the distance in a situation where oncoming vehicles or pedestrians are not present, whereas the driver switches to “low beam” to have the optical axis of the headlight slightly to a lower side so as not to dazzle the drivers of oncoming vehicles and pedestrians in a situation where oncoming vehicles or pedestrians are present.
The brightness and the optical axis of the headlight are test objects of automobile inspection, where an irradiation distance of high beam is generally adjusted to about 100 m towards the front of the vehicle body, and the irradiation distance of low beam is adjusted to about 40 m, and in particular, in a recent high luminance headlight (typically using a metal halide lamp, and normally referred to as an HID lamp), the demand on the adjustment of the bright and dark boundary line (also referred to as a cutoff line) of the beam distribution (particularly low beam) is becoming stricter as even slight optical axis shift greatly dazzles drivers of oncoming vehicles and the like.
FIG. 11 shows a conceptual view of a satisfactory light distribution. In the figure, a vertical line 2 indicating a center of the vehicle, and a horizontal line 3 indicating a height of the headlight of the vehicle (height of the center of optical axis) are drawn on a wall surface 1 positioned at a predetermined distance (10 m herein) to the front of the vehicle (not shown). A hatched portion of the wall surface 1 shows a dark part 4 not illuminated with the headlight, and a white portion other than the hatched portion shows a bright part 5 illuminated with the headlight. A boundary line shown with a heavy light between the dark part 4 and the bright part 5 is a cutoff line 6. The cutoff line 6 spreads slightly to the upper side at the portion on a left side of the vertical line 2 indicating the center of the vehicle, but this is a devise on light distribution so that pedestrians on the left side can be easily found and corresponds to a traveling zone (left-hand traffic) of Japan. Therefore, this devise will obviously be reversed in foreign countries of right-hand traffic.
An aspect to be noted in the figure is that the cutoff line 6 is positioned slightly on the lower side of the horizontal line 3 indicating the height of the headlight excluding a pedestrian portion. Suppose a difference A in the height direction between the horizontal line 3 and the cutoff line is 4.4 cm for convenience, since the wall surface 1 is positioned at 10 m in front of the vehicle, it is 4.4 cm×50 m/10 m=22 cm at the position of 50 m in front of the vehicle, and thus the cutoff line lowers by 22 cm at the 50 m spot. The relevant lowering amount (22 cm) is a sufficient value to prevent dazzling on oncoming vehicles, and also illuminates obstructions at the 50 m spot without trouble.
If a number of passengers or an amount of load of the vehicle are changed even if the cutoff line is appropriately set as above, the cutoff line moves to the upper side, which may dazzle the drivers of oncoming vehicles and the like. This tendency is significant in vehicles such as a minivan, where a great number of seats are provided, and a change in the number of passengers or a change in the load are large.
An auto levelizer (hereinafter referred to as a headlight optical axis adjustment device) for automatically adjusting the optical axis of the headlight to obtain the correct light distribution is thus used. The headlight optical axis adjustment device tilts and adjusts the optical axis of the headlight in a direction of canceling a pitch angle based on the tilt (hereinafter also referred to as a pitch angle) in the front and back direction of the vehicle. In a vehicle mounted with such device, the optical axis of the head light is automatically adjusted to return to the correct light distribution when the number of passengers or the amount of load of the vehicle are changed, and thus the irradiating light beam is prevented from inhibiting other traffics while checking the traffic obstructions at the front of the vehicle at night.
However, the pitch angle of the vehicle finely fluctuates not only when the number of passengers or the amount of load are changed, but also when the passengers goes in/out or when the baggage is being loaded/unloaded to and from the vehicle, and also fluctuates when accelerating or decelerating during traveling or riding over bumps of the road surface. Thus, an optical axis adjustment mechanism of the headlight operates every time the pitch angle changes if the device simply “tilts and adjusts the optical axis of the headlight in a direction of canceling the pitch angle of the vehicle”, and thus operation frequency of actuators such as a motor arranged in the mechanism increases thereby inhibiting durability.
Japanese Unexamined Patent Publication No. 2000-103280 (Japanese Patent No. 3849960) is known as a conventional art for solving such problem. This publication will be hereinafter referred to as a first conventional art, where in the first conventional art, a tilt adjustment of the optical axis of the headlight based on the pitch angle of the vehicle is performed once while the vehicle is stopped, and thereafter, the tilt adjustment is performed for every predetermined interval while the vehicle is stopped. The tilt adjustment of the optical axis of the headlight is thus performed limited to only while the vehicle is stopped, and the number of operations of the actuator such as the motor is reduced and the durability is enhanced.
Japanese Unexamined Patent Publication No. 2000-85458 (Japanese Patent No. 3847972) is also known as another conventional art for solving the above problem. This publication will be hereinafter referred to as a second conventional art, where in the second conventional art, the tilt adjustment of the optical axis of the headlight based on the pitch angle of the vehicle is performed while the vehicle is stopped, and the title adjustment of the optical axis of the headlight is performed based on the pitch angle of when stably traveling instead of the pitch angle of when the vehicle is stopped if the difference between the pitch angle of when stably traveling and the pitch angle of when stopped exceeds a predetermined value while the vehicle is stably traveling (constant speed travel that does not involve acceleration and deceleration of greater than or equal to 30 Km per hour). In this case as well, the tilt adjustment of the optical axis particularly in traveling is limited to being performed only “if the difference between the pitch angle of when stably traveling and the pitch angle of when stopped exceeds a predetermined value”, and thus the number of operations of the actuator such as the motor is reduced and the durability is enhanced.